To be able to change the frequency, we have to change to the userspace frequency.

sudo cpufreq-set -g userspace

Finally, we can switch the actual frequency to one of the available values with:

sudo cpufreq-set -f 2.6GHz

This happend per-core on the server I tested the procedure on. You can select e.g. CPU 2 using the -c2 option. Also, don’t forget to change back to the default ondemand governor once you’re done – you wanna stay green 😉